1. Field of the Invention
The present invention relates to an ohmic electrode for use in an optical device and a fabrication method thereof, and more particularly, to a p-type ohmic electrode in a gallium nitride based(GaN based) optical device and a fabrication method thereof.
2. Description of the Related Art
Optical devices, e.g., light emitting devices utilizing gallium nitride based compound semiconductors such as GaN, InGaN or AlGaN, effectively emit light with blue light wavelength region and have high light power compared to conventional optical devices. In particular, blue light has a shorter wavelength than red light, it can be easily converted into longer wavelength light. Thus, in recent years, a great deal of attention has been directed to GaN light emitting devices as white-light emitting devices.
In fabricating a GaN light emitting device, an n-type ohmic electrode and a p-type ohmic electrode are formed on an n-type gallium nitride (n-GaN) layer and a p-type gallium nitride (p-GaN) layer, respectively. The n-type and p-type ohmic electrodes must have low contact resistance and must be thermally stable. In particular, the p-type ohmic electrode must be a transparent electrode that can transmit emitted light. However, the p-type ohmic electrode formed on the p-GaN layer has poor activation efficiency of Mg dopant doped into the p-GaN layer, so that a hole concentration of 1018 cm−3 or higher cannot be obtained, making it difficult to attain contact resistance as low as approximately 10−5·cm2.
In a GaN light emitting device, a nickel (Ni)/gold (Au) layer formed by stacking an Ni layer on an Au layer is generally used as a p-type ohmic electrode because contact resistance of the ohmic electrode after annealing is low, e.g., approximately 10−4·cm2, the Ni layer is easily converted into a transparent nickel oxide (NiO) layer that easily transmits light, and the Au layer increases lateral transmittance of the electrode. The Ni/Au ohmic electrode having the entire thickness of approximately 10 nm has superior transparency to light in a wavelength region of 300.500 nm, that is, approximately 80%. However, when the Ni/Au ohmic electrode is annealed for forming an ohmic contact, internal diffusion of Au may occur, suggesting poor thermal stability, and the lateral profile of the Au layer is not uniform.